The pattern of methylation of genomic DNA becomes significantly altered during oncogenesis. In human tumors, the overall level of DNA-cytosine methylation is decreased, but CpG islands generally become hypermethylated, resulting in frequent epigenetic inactivation of tumor suppressor genes. One of the most common observations is that one allele of a tumor suppressor gene becomes mutated or lost, and the other allele becomes silenced by hypermethylation. Little is known of the events that trigger this aberrant de novo methylation, nor of is it known which of the three characterized DNA methyltransferases is responsible. It is however, well recognized that the methylation machinery must be under strict regulatory control during development, and that the balance of factors involved in this regulation appears to be disrupted during tumorigenesis. The tumor suppressor gene, p53 is mutated or lost in more than 50% of all types of human tumors, and appears to be an early event in tumorigenesis in many cases. We have recently found that p53 binds the DNA methyltransferase 1 (Dnmt1) promoter in the absence of stimuli that activate p53, that activation of p53 reduces this binding, and that loss of p53 function induces upregulation of Dnmt1. These data suggest that aberrant genomic methylation might be promoted by alteration or loss of this important cancer gene. We now propose to determine the underlying mechanism of p53-mediated control of DNA methyltransferase1, its generality to the related DNA methyltransferases, and its role in the loss of tumor suppressor gene function during carcinogenesis. We propose to study the interaction of wild type and mutant p53s with the promoter regions of the Dnmt loci both in vitro and in vivo, using chromatin immunoprecipitation, to modulate the levels of p53 and Dnmts using siRNA technology, and to study the composition of protein complexes involving p53 and its mutant forms resident on the promoters of the Dnmt genes using tandem mass spectrometry. These studies will lead to an understanding of the early events in tumor progression that result in disruption of the control of DNA methylation. The inappropriate tumor suppressor gene silencing that follows this loss of control appears to be critical in promoting oncogenesis.